1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly relates to a constitution of a pair of sheet conveying rollers provided in a sheet conveying portion.
2. Description of the Related Art
In the prior art, an image forming apparatus including a printer, a copying machine, and a FAX generally includes a registration portion for use in correction of skew feeding of a sheet conveyed to an image forming portion and positional alignment between an image formed in the image forming portion and a sheet. As such a registration portion, there is one which corrects the skew feeding of a sheet by means of a pair of registration rollers and aligns the positions of an image formed in the image forming portion and a sheet (see, U.S. Pat. No. 6,889,031). In the registration portion including the pair of registration rollers, first, the front end of a sheet, conveyed by a pair of conveying rollers on the upstream side, is butted against a nip of the pair of stopped registration rollers, whereby the front end of the sheet is follows the nip line of the pair of registration rollers, and the skew feeding of the sheet is corrected. Then, a timing of starting the rotation of the pair of registration rollers is controlled, whereby the positional alignment between the image formed in the image forming portion and the sheet are performed.
In the prior art image forming apparatus including the registration portion, the sheet subjected to the skew feeding correction is conveyed to the image forming portion by the rotation of the pair of registration rollers; however, at that time, the rear end of the sheet first passes through a nip of the pair of conveying rollers of upstream and thereafter to pass through a nip of the pair of registration rollers. When the rear end of a sheet passes through the nip of the pair of conveying rollers, which is a pair of sheet conveying rollers, and the nip of registration roller pair, an acceleration occurs in the sheet. The velocity of conveying the sheet is varied upon the occurrence of the acceleration in the sheet. Especially, in a case of using a thick and stiff sheet such as cardboard, in order to stably convey the sheet without causing a slip of the sheet, a conveying force from a strong conveying nip is required. However, the stronger the conveying nip, the larger an impact that occurs when the sheet passes through the nip. The variation of the sheet conveying velocity may cause a deviation of an image formed on a sheet in an image forming portion.
Thus, in view of the above problems, the present invention provides an image forming apparatus which can suppress a variation of a velocity of conveying a sheet that occurs when the sheet passes through a pair of sheet conveying rollers.